Were the lights there Fluorescent?Only they actually flash on and off at twice the mains frequency, as opposed to filament bulbs which dont.I've had it before, where my D3, has done exactly the same,Its to do with the flicker of the lighting system, and you will probably find that the speed of the shutter was much faster then the flicker of the lights, either 100hz or 120hz (depending if its UK 50Hz or USA60Hz) so what happens is you actually managed to shoot when the light was technically on, in one photo and then when it was technically off, in the other. The D4 being as quick as it is, managed to change the white balance to suit.The only way round this is to manually set the white balance to match the lighting.Mac.